The effects of geometry, elasticity, and external pressures on the diastolic pressure-volume and stiffness-stress relations. How important is the pericardium?

The concept of an incremental elastic modulus is applied in the quantification of passive elastic stiffness-stress relations of intact heart muscle, and a transmural pressure-volume relation for the left ventricle is subsequently derived in terms of geometry, muscle elasticity, and external pressures to assess their importance. Physiological and clinical applications of this method indicate that: (1) stiffness-stress relations obtained on the basis of pressure-volume data from dog hearts are not significantly different from those obtained from muscle strips excised from these same hearts; (2) shape and the presence of right ventricular, pericardial, or pleural pressures are of secondary importance in an assessment of passive elastic stiffness; and (3) dramatic shifts in the left ventricular intracavity pressure-volume relations following drug interventions are primarily due to the presence of substantial pericardial pressures; however, the transmural pressure-volume relations are not markedly altered, implying no alteration in the intrinsic ventricular compliance.